Some image-bearing materials are subjected to further manufacturing processes after a pattern is applied to the material. These additional processes can create topical--regional--distortions in the material such that the material and thus the pattern varies nonuniformly across the length and breadth of the material. Some such materials are fabrics and printed wire boards. Difficulties arise when the quality of the patterns must be assured.
Presently, printed wire boards are inspected using time-consuming and expensive processes. Most of the inspection is performed manually; this method can amount to thirty to forty percent of the entire cost of the board.
The most common automatic inspection systems utilize the design rule approach. Under this approach, the inspection system examines a small area of the board to determine if the circuit fit certain design rules. For example, acceptable lines have a given uniform width, have a right-angle bend, or end in a pad. Errors are identified when a pattern does not match the rules: a line ending without a pad is an error, indicating that an open circuit exists; a line ending in a line is an error, indicating a short circuit. As designs increase in complexity, more rules are required to accurately describe acceptable circuits and computational demands increase. State of the art printed wire boards require a great deal of time for inspection by conventional design rule systems. Further, a change in design often necessitates alteration of the design rules.
Under another automatic inspection system, the golden board approach, a perfect or "golden" board is used as the standard to which production printed wire boards are compared. The golden board may be compared in its hardware embodiment or in the form of a data base, that is, a computer map of the golden board. The major problem with the golden board approach is that production printed wire boards typically exhibit shrinking or stretching distortions imparted by the manufacturing process which are nonuniform over the length of the production board. Circuit material typically varies in length and width 0.0005 inch per inch or up to 0.01 inch over 24 inches. While rarely rendering a production board defective, these topical distortions often prevent direct area-to-area matching between entire full-sized boards.
Further, complex boards can exert memory demands of nearly two gigabits to conventionally represent an entire printed wire board in a data base. The time required simply to retrieve and manipulate this vast quantity of information to accommodate the particular variations of a given board renders conventional golden board systems too slow to be economically feasible.